1. Field of the Invention
The present invention relates to the area of integrated circuits, and more particularly to a circuit for trimming a current source.
2. Description of Related Art
Current sources may be found in various integrated circuits (IC), such as DC/DC converters. An accurate current source helps improve the electrical performance and also helps to increase the yield in fabrication with small variation. In addition, a designer often requires a highly accurate output current so that an implementation using the current could be made relatively easier. However, it is not trivial to create an accurate current source on a chip without external components because on-chip component values often change.
In the state of the art, two methods are often used to control a current source. One of them is to allocate a special pin and connect it to an external accurate resistor. An internal voltage buffer is implemented to regulate the current flowing through the resistor. In many cases, however, an allocation of this special pin is not practical in many discrete analog devices. Another method is to design an on-chip trimming circuit. The process variations may be corrected by the trimming circuit after fabrication. Some designs adopt on-wafer trimming while others choose after-package trimming. Both of them have some inherent drawbacks. The on-wafer trimming might experience a serious shift after package. Furthermore, some trimming techniques like metal-fuse trimming and poly-fuse trimming may lead to reliability Issues. The main problem of the after-package trimming is the additional cost because design complexity increases die size and needs more design effort. Therefore, a simpler circuit structure or trimming method is in demand. Further, flexibility in a trimming technique is also needed so that a resulted trimming current value may be adjusted by an end user.